John T. Turk

Processes Controlling the Chemistry of Two Snowmelt‐Dominated Streams in the Rocky Mountains

Time-intensive discharge and chemical data for two alpine streams in the Loch Vale watershed, Colorado, were used to identify sources of runoff, flow paths, and important biogeochemical processes during the 1992 snowmelt runoff season. In spite of the paucity of soil cover the chemical composition of the streams is regulated much as in typical forested watersheds. Soils and other shallow groundwater matrices such as boulder fields appear to be more important in controlling surface-water chemistry than their abundance would indicate. The chemical composition of the major source waters (usually thought of as end-members whose chemical composition is relatively constant over time) changes at the same time that their mixing ratio in streams changes, confounding use of end-member mixing models to describe stream-water chemistry. Changes in the chemical composition of these source waters are caused by the ionic pulse of solutes from the snowpack and the small size of the shallow groundwater reservoir compared to the volume of snowmelt passing through it. The brief hydrologic residence time in the shallow groundwater indicates that concentrations of most dissolved constituents of stream water were controlled by fast geochemical processes that occurred on timescales of hours to days, rather than slower processes such as weathering of primary minerals. Differences in the timing of snowmelt-related processes between different areas of the watershed also affect the stream-water chemical composition. Cirque lakes affect discharge and chemical composition of one of the streams; seasonal control on stream-water NO3 and SiO2 concentrations by diatom uptake in the lakes was inferred. Elution of acidic waters from the snowpack, along with dilution of base cations originating in shallow groundwater, caused episodes of decreased acid-neutralizing capacity in the streams, but the streams did not become acidic.

STRONTIUM 87/STRONTIUM 86 AS A TRACER OF MINERAL WEATHERING REACTIONS AND CALCIUM SOURCES IN AN ALPINE/SUBALPINE WATERSHED, LOCH VALE, COLORADO

Sr isotopic ratios of atmospheric deposition, surface and subsurface water, and geologic materials were measured in an alpine/subalpine watershed to characterize weathering reactions and identify sources of dissolved Ca in stream water. Previous studies have noted an excess of Ca in stream water above that expected from stoichiometric weathering of the dominant bedrock minerals. Mixing calculations based on 87 Sr/ 86 Sr indicate that on an annual basis, 26 67% of Ca export in streams is atmospherically derived, 23 61% is from weathering of plagioclase, and the remainder is from weathering of calcite present in trace amounts in the bedrock. A potential source of error when applying Sr isotopes in catchment studies is determination of the 87 Sr/ 86 Sr of Sr released by mineral weathering, which is complicated by the wide range of mineral isotopic compositions, particularly in older rocks, and the variable rates at which the minerals weather. In this study, base-flow stream chemistry was used to represent the 87 Sr/ 86 Sr of Sr derived from mineral weathering because it effectively integrates the potentially variable isotopic composition of Sr released by weathering in the alpine environment.

Environmental characteristics and water quality of hydrologic benchmark network stations in the west-central United States, 1963-95

The Hydrologic Benchmark Network was established in 1963 to provide long-term measurements of streamflow and water quality in areas that are minimally affected by human activities. These data were used to study time trends and to serve as controls for separating natural fron1 artificial changes in other strean1s. The network has consisted of as many as 58 drainage basins in 39 States. This report describes the environn1ental characteristics and water quality at 12 benchmark basins in the Western United States. The stations discussed in this report and their physiographic provinces are as follows: Wet Bottom Creek, Arizona, in the Southern Rocky Mountains; Elder Creek, California, in the Pacific Border Province; Merced River and Sagehen Creek, California, and Crater Lake, Oregon, in the Sierra-Cascade Mountains; Big Jacks Creek, Idaho, and Minam River, Oregon, in the Columbia Plateaus; Hayden Creek, Idaho, and Andrews Creek, Washington, in the Northern Rocky Mountains; South Twin River and Steptoe Creek, Nevada, in the Basin and Range; and Red Butte Creek, Utah, in the Middle Rocky Mountains. The information in this report was compiled to aid in the application and interpretation of historical water-quality data collected by the U.S. Geological Survey Hydrologic Benchmark Network program. The Hydrologic Benchmark NetT;vork streams discussed in this report drain either forested areas or grasslands and have land-use activities that include recreational use, timber harvesting, grazing, and scientific research. Surface-water chemistry in the benchmark basins was controlled by the interaction of dilute precipitation with the underlying soils and bedrock, and land-use activities seemed to have a minimal effect on the major-ion and nutrient chemistry of stream water at the Hydrologic Benchmark Network stations. Temporal trends in waterquality constituents were observed at a number of the stations and were attributed to environmental and method-related factors. Trends in base cations and alkalinity at Elder Creek and Sagei.en Creek, California, and Red Butte Creek, Utah, seemed to be associated with an extended period of drought that persisted fron1 the late 1980s through the early 1990s in both regions of the country. Statistically significant upward trends in field pH and downward trends in sulfate that were observed at several of the stations were attributed to changes in field instrumentation and analytical techniques rather than environmental change.

Comparison of snowpack and winter wet-deposition chemistry in the Rocky Mountains, USA: implications for winter dry deposition

Depth-integrated snowpack chemistrywas measured just prior to maximum snowpack depth during the winters of 1992–1999 at 12 sites co-located with National Atmospheric Deposition Program/National Trend Network (NADP/ NTN) sites in the central and southern RockyMountains, USA. Winter volume-weighted mean wet-deposition concentrations were calculated for the NADP/NTN sites, and the data were compared to snowpack concentrations using the paired t-test and the Wilcoxon signed-rank test. No statisticallysignificant differences were indicated in concentrations of SO4� or NO3 (p > 0:1). Small, but statisticallysignificant differences ( pp0:03) were indicated for all other solutes analyzed. Differences were largest for Ca 2+ concentrations, which on average were 2.3meq l � 1 (43%) higher in the snowpack than in winter NADP/NTN samples. Eolian carbonate dust appeared to influence snowpack chemistrythrough both wet and drydeposition, and the effect increased from north to south. Drydeposition of eolian carbonates was estimated to have neutralized an average of 6.9meq l � 1 and a maximum of 12meq l � 1 of snowpack acidityat the southernmost sites. The good agreement between snowpack and winter NADP/NTN SO 4� and NO3 concentrations indicates that for those solutes the two data sets can be combined to increase data densityin highelevation areas, where few NADP/NTN sites exist. This combination of data sets will allow for better estimates of atmospheric deposition of SO4� and NO3 across the RockyMountain region. Published byElsevier Science Ltd.

Timescales for migration of atmospherically derived sulphate through an alpine/subalpine watershed, Loch Vale Colorado

Sulphur 35, a cosmogenically produced radioisotope with a short half-life (87 days), was measured in snowpack during 1993–1997 and at four locations within the Loch Vale watershed during 1995–1997. The four sites include the two main drainages in the watershed, Andrews Creek and Icy Brook, a small south facing catchment flowing into Andrews Creek (Andrews Spring 1), and a similar north facing catchment flowing out of a scree field into Icy Brook (Spring 19). Concentrations ranged from a high of almost 50 mBq/L for a sample from Spring 19 in June 1996 to a concentration near the detection limit for a sample from Andrews Creek in April 1997. Sulphur 35 concentrations were normalized to sulphate (as mBq/mg SO4−2) and were decay-corrected to a Julian day of 90 (April 1) for each year. Snowpack had the highest 35S concentration with an average concentration of 53 mBq/mg SO4−2. Concentrations in the streams were much lower, even when corrected for decay relative to JD 90. The large 35S concentrations found in Spring 19 were the result of increases in concentration due to sublimation and/or evapotranspiration and were lower than snowpack when normalized to sulphate. Using 35S concentrations found in snowpack as of JD 90 as a beginning concentration, the fraction of sulphate in streamflow that was derived from atmospheric deposition within the prior water year was estimated. For Icy Brook and Andrews Creek the fraction of the sulphate in streamflow derived from that years snowpack and precipitation was low prior to the beginning of the main spring melt, reached a maximum during the period of maximum flow, and decreased as the summer progressed. A calculation of the seasonal flux indicated that about 40% of the sulphate that flowed out of the watershed was derived from atmospheric sulphate deposited during the previous year. This suggests that more than half of the sulphate deposited in the watershed by atmospheric processes during the previous year was removed during the following summer. Thus sulphate retention in alpine watersheds like Loch Vale is very limited, and changes in sulphate deposition should be quickly reflected in stream chemistry.

Use of cosmogenic 35S for comparing ages of water from three alpine–subalpine basins in the Colorado Front Range

Abstract High-elevation basins in Colorado are a major source of water for the central and western United States; however, acidic deposition may affect the quality of this water. Water that is retained in a basin for a longer period of time may be less impacted by acidic deposition. Sulfur-35 ( 35 S ), a short-lived isotope of sulfur ( t 1/2 =87 days), is useful for studying short-time scale hydrologic processes in basins where biological influences and water/rock interactions are minimal. When sulfate response in a basin is conservative, the age of water may be assumed to be that of the dissolved sulfate in it. Three alpine–subalpine basins on granitic terrain in Colorado were investigated to determine the influence of basin morphology on the residence time of water in the basins. Fern and Spruce Creek basins are glaciated and accumulate deep snowpacks during the winter. These basins have hydrologic and chemical characteristics typical of systems with rapid hydrologic response times. The age of sulfate leaving these basins, determined from the activity of 35 S , averages around 200 days. In contrast, Boulder Brook basin has broad, gentle slopes and an extensive cover of surficial debris. Its area above treeline, about one-half of the basin, is blown free of snow during the winter. Variations in flow and solute concentrations in Boulder Brook are quite small compared to Fern and Spruce Creeks. After peak snowmelt, sulfate in Boulder Brook is about 200 days older than sulfate in Fern and Spruce Creeks. This indicates a substantial source of older sulfate (lacking 35 S ) that is probably provided from water stored in pore spaces of surficial debris in Boulder Brook basin.

Use of chemistry and stable sulfur isotopes to determine sources of trends in sulfate of Colorado lakes

The chemistry of lakes in the Mt. Zirkel Wilderness Area (MZWA) and the Weminuche Wilderness Area (WWA) of Colorado has been monitored since 1985. The initial results indicate that changes have occurred in the chemistry of some lakes in both areas. Increased concentration of sulfate in lakes may be related to increased atmospheric depositon of sulfate or to changes of sulfate released by weathering and to changing dilution of sulfate by snowmelt. Stable S isotopes seem to be capable of separating the fraction of change in sulfate that is related to atmospheric and watershed sources. Because of the short period of record, it is not possible to determine whether the changes are part of a long-term trend or are merely natural fluctuations about some baseline.

Use of Natural 35S to Trace Sulphate Cycling in Small Lakes, Flattops Wilderness Area, Colorado, U.S.A.

Measurements of the cosmogenically-produced 35S, a radioisotope of sulphur (t1/2 = 87 days), are reported for the Ned Wilson Lake watershed in Colorado. The watershed contains two small lakes and a flowing spring presumed to be representative of local ground water. The watershed is located in the Flattops Wilderness Area and the waters in the system have low alkalinity, making them sensitive to increases in acid and sulphate deposition. Time series of 35S measurements were made during the summers of 1995 and 1996 (July–September) at all three sites. The system is dominated by melting snow and an initial concentration of 16–20 mBq L-1 was estimated for snowmelt based on a series of snow samples collected in the Rocky Mountains. The two lakes had large initial 35S concentrations in July, indicating that a large fraction of the lake water and sulphate was introduced by meltwater from that years snowpack. In 1995 and 1996, 35S concentrations decreased more rapidly than could be accounted for by decay, indicating that other processes were affecting 35S concentrations. The most likely explanation is that exchange with sediments or the biota was removing 35S from the lake and replacing it with older sulphate devoid of 35S. In September of 1995 and 1996, 35S concentrations increased, suggesting that atmospheric deposition is important in the sulphate flux of these lakes in late summer. Sulphur-35 concentrations in the spring water were highly variable but never higher than 3.6 mBq L-1 and averaged 2 mBq L-1. Using a simple mixing model, it was estimated that 75% of the spring water was derived from precipitation of previous years.

Response of Ned Wilson Lake Watershed, Colorado, to Changes in Atmospheric Deposition of Sulfate

The Ned Wilson Lake watershed responds directly and rapidly to changes in precipitation inputs of sulfate, which has important implications for effects of acid deposition on the aquatic system. Chemistry at three precipitation collection sites and three watershed sites (a pond, a lake, and a spring) has been monitored in and near the Flattops Wilderness Area in northwestern Colorado beginning in 1981–1983. Bulk snowpack concentration of sulfate in the watershed and volume-weighted annual mean concentration of sulfate in precipitation at two nearby sites generally decreased from 1981 to 1985, were small through 1987, and increased in 1988–1989. Changes in concentration of sulfate at the watershed sites are controlled by precipitation inputs. Responsiveness of the individual sites was dependent on their position along the hydrologic flow path. The fastest response was in the pond, which has a hydrologic residence time of less than 1 year; over 90% of the variance in concentration of sulfate in the pond was explained by changes in concentration in precipitation. The lake has a hydrologic residence time of 1 to 4 years; a regression model of the concentration of sulfate in the lake, as a function of the concentration in the lake during the previous year and the concentration in precipitation, explained 87% of the variance in concentration of sulfate in the lake. The hydrologic response time of the spring is unknown; it was not responsive to changes in concentration of sulfate in precipitation. The recent increase of sulfate concentration in precipitation and in the pond and lake is evidence for a rapid rather than a delayed response, which could not be determined when only a decreasing trend in sulfate concentration was reported in 1982–1987. Watersheds of this type are sensitive to acidification (acid-neutralizing capacity less than 60 μeq L−1), and these results indicate conservative behavior of sulfate. This is important in predicting effects of future changes in atmospheric deposition, which could potentially be caused by anthropogenic emissions or climatic change.

Controls on the major ion chemistry of the Ürümqi River, Tian Shan, People's Republic of China

Water and snow samples were collected in May of 1990 and 1992 in headwater basins and along a longitudinal transect of the Urumqi River, located in northwestern China. Surface waters were dominated by Ca2+ and HCO3− at all sites. Maximum measured SO42− concentrations in surface waters were 550μegL−1, and were balanced by Ca2+ and HCO3−; pH was slightly alkaline at all locations. Several independent analyses each concluded that the solute composition of surface waters was dominated by dissolution of rocks with rapid weathering kinetics, such as calcite and dolomite. Preliminary analysis of stable sulfur ratios in headwater basins shows δ34S values of +6.8 for snow and +3.3 for surface waters. Changes in the δ34S value of surface waters with increasing basin area were variable, suggesting changes in the stable sulfur ratios of source materials. The large amounts of HCO3− and base cations at all sites indicates that the Urumqi River is not sensitive to acidification from atmospheric deposition.